1. Field of the Invention
The present invention relates to a method for producing 2-chloropyridine and 2,6-dichloropyridine by photochemically chlorinating pyridine in a gas phase. Also, the present invention relates to a method for efficiently separating 2,6-dichloropyridine from the aqueous reaction mixture containing 2,6-dichloropyridine, 2-chloropyridine and pyridine obtained by photochemical chlorination of pyridine using water as a diluent or thermal chlorination of pyridine. Both 2-chloropyridine and 2,6-dichloropyridine are important intermediates for medicines and agricultural chemicals.
2. Discussion of the Related Art
There are two methods known for obtaining 2-chloropyridine and 2,6-dichloropyridine by pyridine chlorination: thermal chlorination of pyridine at high temperatures of 370.degree. to 430.degree. C. (DE2208007) and photochemical chlorination of pyridine within the temperature range from about 78.degree. to 125.degree. C. under irradiation with a high pressure mercury lamp (U.S. Pat. No. 3,297,556).
Because the thermal chlorination method is carried out under high temperature conditions, condensation reactions of the resulting 2-chloropyridine and 2,6-dichloropyridine with the starting material pyridine occur to form tar, which in turn lowers the yield of the desired products. This method poses another problem, which is the difficulty of purification of the desired products from the reaction mixture due to the presence of a large amount of impurities in the reaction mixture. Also, since 2,6-dichloropyridine, one of the desired products, begins to decompose exothermally near 370.degree. C. and decomposes rapidly at 380.degree. C., as determined by differential thermal analysis, the thermal chlorination method is not preferable for obtaining 2,6-dichloropyridine.
On the other hand, the photochemical chlorination method is usually carried out in a gas phase by using a high pressure mercury lamp as a source of ultraviolet rays (UV), wherein pyridine, chlorine and a diluent, all in a vaporized state, are exposed to UV irradiation. Conventionally, there are two choices for the diluent, i.e., a halogenated hydrocarbon or water. When a halogenated hydrocarbon is used as the diluent, the above-mentioned problem of tar formation is unlikely to occur because the reaction temperature is usually 160.degree. to 190.degree. C., lower than that for the thermal chlorination method, and, therefore, a reaction mixture without impurities can be obtained. When water is used as the diluent, reaction temperature is also low, usually 160.degree. to 170.degree. C.
However, the heat of reaction in 2-chloropyridine production from pyridine is about 30 kcal/mol, and that in 2,6-dichloropyridine production from pyridine is about 60 kcal/mol; both reactions are highly exothermic. For this reason, compounds which do not undergo chlorination per se, such as carbon tetrachloride having a great molar specific heat, have been used as diluents for the reaction in order to remove such heat of reaction and maintain a low reaction temperature. However, the use of carbon tetrachloride is becoming difficult due to its carcinogenicity and the recent legal regulation on FLON (a chlorofluorocarbon product) and HALON (bromochlorofluorocarbons and bromochlorocarbons). Although other various halogenated hydrocarbons have been proposed as substitutes for carbon tetrachloride, none is free from the legal regulation on Flon and Halon and the problem of carcinogenicity. Accordingly, various methods using water as the diluent have recently been proposed.
Because the molar specific heat of water is much lower than that of gaseous carbon tetrachloride, removal of heat of reaction using steam requires more molar amount than that of carbon tetrachloride, as a diluent for the reaction. However, increasing the amount of diluent is disadvantageous because the reaction itself is decelerated due to dilution of pyridine and chlorine with the steam, though heat removing capability based on sensible heat increases. There is also a problem that removal of heat of reaction becomes difficult due to poor heat transfer through the reactor wall when the reaction is carried out in a gas phase. Thus, the conventional photochemical chlorination reaction is faulty in that experimental conditions cannot be applied directly to an increased scale of reaction. This is true not only when steam is used as a diluent, but also when carbon tetrachloride is used as a diluent. In other words, increasing the reactor size results in a decreased heat-transfer surface area per unit volume and hence a decreased total amount of heat removed through the reactor wall.
In the above situation, Japanese Patent Examined Publication Nos. 55-4742 and 52-3935 and Japanese Patent Laid-Open Nos. 1-207270 and 1-308256 propose methods using water as a diluent. In the preferred embodiments of these methods shown in the respective specifications, reactor capacity is 1 to 5 liter, remaining within the range of laboratory scale or intermediate experimental scale. On such scale, heat-transfer surface area per unit volume of the reactor is sufficient to remove heat of reaction, and photochemical chlorination is carried out usually at 160.degree. to 170.degree. C. as stated above by external cooling.
However, unlike laboratory-scale production, industrial production of 2-chloropyridine and 2,6-dichloropyridine requires a reactor capacity of at least 100 liters, usually 300 liters or more. On this scale, removal of the heat of reaction is very difficult because heat-transfer surface area per unit volume of the reactor decreases drastically.
Also, uniformly mixing three components, namely pyridine, chlorine and a diluent in a gas phase poses an important problem from the viewpoint of chemical reaction engineering. Specifically, localization of high concentration chlorine results in a local rise in reaction temperature, leading to an uneven distribution of high temperature portions in the reactor. Also, the resulting 2,6-dichloropyridine may undergo further chlorination by the action of the high concentration chlorine, which can increase amounts of by-products such as trichloropyridine and tetrachloropyridine.
As a solution to these problems, industrial production may be carried out using a number of small reactors, but such approach is unrealistic due to troublesome instrumentation and piping associated with the increased number of reactors.
Also, the amount of steam may be increased to remove heat of reaction and to dilute chlorine. In this method, however, excessive dilution lowers the chlorine and pyridine concentrations, resulting in decreased production per unit time. Moreover, this method is undesirable not only because isolation and purification of 2-chloropyridine and 2,6-dichloropyridine become difficult but also because increased discharge of waste water may become a problem in view of environmental protection.
Alternatively, reaction temperature itself may be set low to compensate for the occurrence of local hot portions due to uneven distribution of chlorine concentration, but this method is not industrially advantageous because of decreased productivity.
Concerning the separation of 2,6-dichloropyridine from the reaction mixture containing 2,6-dichloropyridine, 2-chloropyridine and pyridine obtained by photochemical chlorination or thermal chlorination of pyridine, various methods have been disclosed. Above all, the particularly efficient method is described in Japanese Patent Laid-Open No. 3-58971. This method separates and purifies 2,6-dichloropyridine from a reaction mixture of 2,6-dichloropyridine, 2-chloropyridine and pyridine by distillation in the presence of hydrogen chloride in a still.
However, in the method of Japanese Patent Laid-Open No. 3-58971, a high concentration of hydrogen chloride must be maintained during distillation to facilitate separation of 2,6-dichloropyridine by distillation. For maintaining such a high concentration of hydrogen chloride, a large amount of hydrochloric acid or sulfuric acid, depending on the composition of reaction mixture in the distillation still, must be added. In other words, the amount of hydrochloric acid added to the reaction mixture is determined by the degree of chlorination of pyridine. When it is desired to obtain 2,6-dichloropyridine in a larger amount than that of 2-chloropyridine by an increased degree of chlorination, the amount of hydrochloric acid formed increases, so that only a small amount of hydrochloric acid is required for distillation. However, when it is desired to obtain 2-chloropyridine in a larger amount than that of 2,6-dichloropyridine by a decreased degree of chlorination, a large amount of hydrochloric acid must be added because the amount of hydrochloric acid formed decreases. Also, for recovering 2-chloropyridine and pyridine from the reaction mixture after 2,6-dichloropyridine distillation, the reaction mixture must be adjusted to an appropriate pH level, which in turn requires the addition of alkali in an amount equivalent to the amount of acid added.
As stated above, the method of Japanese Patent Laid-Open No. 3-58971 is not satisfactory from economic viewpoint because a large amount of acid is required to be added at steam distillation, depending on the reaction mixture composition, and the acid must be neutralized in a series of recovery processes up to the recovery of 2-chloropyridine and unreacted pyridine, which in turn requires an increased amount of alkali.
Moreover, in the above method, a mixture of 2,6-dichloropyridine as an oil and water is obtained as a distillate, which contains hydrochloric acid and a small amount of 2-chloropyridine. Since this distillate can be separated into two layers, namely a water layer and an oil layer, and since the hydrochloric acid and 2-chloropyridine are mostly contained in the water layer, they are mostly removed along with water by liquid separation. However, a small amount of hydrochloric acid and 2-chloropyridine may be present in the oil layer containing 2,6-dichloropyridine, which can deteriorate the quality of 2,6-dichloropyridine. For this reason, the 2,6-dichloropyridine must be further treated by washing, etc. to obtain 2,6-dichloropyridine of high purity.